1. Technical Field
This invention relates to equalization systems for use in digital communications and more specifically to an equalization system for compensating for undesirable degradations that occur to signals in communication channels.
2. Description of the Prior Art
Digital communication systems which employ digitally modulated signals are increasingly being used in wireless and cellular applications. While propagating through a communication channel, these digitally modulated signals incur degradations in the form of intersymbol interference, fading and noise. These undesirable degradations on the signal are generally compensated for through use of equalization provided by equalizers in receivers employed in the digital communication systems.
The operation of conventional equalizers is well known and described, for example, by S. U. Quereshi in an article entitled Adaptive Equalization, Proceedings of the IEEE, Vol. 73, pages 1349-1387, September 1985. Two types of equalizer structures are most often used, transversal filters and Viterbi structures. Transversal filters consist of tap delay lines through which the received digitally modulated signals are shifted and the output of every tap is multiplied by a different coefficient. The output of the transversal filter based equalizer is the summation of all of the outputs from the taps multiplied by the coefficients. A part of transversal equalizers is a coefficient adaptation process which adjusts the coefficients such that the output of the equalizer is a signal without intersymbol interference degradation.
Viterbi equalizers are fundamentally different from transversal equalizers. Viterbi equalizers detect the sequence of the received signal for obtaining the maximum likelihood estimate of the sequence of the transmitted signal. The trellis, which is used for Viterbi equalization, represents all possible sequences of the transmitted signal. In operation, the Viterbi equalizer identifies a path through the trellis, this path representing the most likely sequence of the transmitted signal.
The properties of radio propagation through multipath fading channels, and the limitations of receiver technology impose a number of requirements on equalizers which attempt to compensate for degradations that occur to digitally modulated signals while in these channels. Traditional equalization structures and coefficient adaptation methods have disadvantages. By way of illustrative example, a numeration of the most often used equalizers and some particular disadvantages of these equalizers is provided. Linear transversal equalizer structures have noise inversion problems. Decision feedback equalizer (DFE) structures with least mean squares (LMS) coefficient adaptation have slow coefficient convergence. DFE structures with recursive least squares (RLS) adaptation have high computational complexity and are numerically unstable. Lattice equalizer structures are computationally complex. Tap-jamming based transversal equalizers are characterized by memory inefficient block processing and high computational complexity. And maximum likelihood sequence estimation (MLSE) equalization techniques have a high computational complexity and a need to have good channel estimates. Also, these MLSE equalization techniques are sensitive to phase errors and have high memory requirements.
As apparent from the above, various techniques of equalization are known and have been used to date. These techniques are, however, either computationally complex or are limited in performance in some manner. It is therefore desirable that equalization techniques not only be capable of satisfactory performance but also be computationally simple. With an increased need for portable communication terminals, such as digital cellular telephones and cordless telephones, there is an increased need for this type of equalization technique. The availability of such a technique advantageously would allow, for example, power consumption to be conserved in these portable communication terminals which, while in their normal operating environment, receive all operating power from a battery usually contained within the terminal.